Senator Claire McCaskill, once regarded as the chamber’s most vulnerable incumbent Democrat, won a second term on Tuesday, buoyed by voters concerned about comments made by her Republican opponent, Representative Todd Akin, that women who are victims of “legitimate rape” rarely become pregnant.

Exit polls showed that women outnumbered men at the polls, 55 percent to 45 percent, and favored Ms. McCaskill by an even larger margin in one of the most closely watched and contentious Senate races in the country.

Republicans had once counted on winning Ms. McCaskill’s seat, but that changed in August when Mr. Akin’s comments squarely positioned abortion as a defining issue in Missouri and around the nation.

It was the type of moment Ms. McCaskill was hoping for when she and Democrats spent nearly $2 million for advertising during the Republican primary bolstering Mr. Akin over other candidates, with the hope of depicting him as an extremist afterward. In the public fury that followed the comment, Republicans urged Mr. Akin to quit the race and stripped him of financial support. Mr. Akin apologized for his remark but refused to withdraw.

Analysts had expected Mr. Akin’s comments to be particularly costly among Republican women in the more affluent suburbs of St. Louis and Kansas City, who may either have ignored the race altogether or split their ticket. In exit polls, 64 percent of voters said Mr. Akin’s comments were either the most important factor or one of several important factors in the race. Nearly a quarter of those voters identified themselves as Republicans.

In the race for the White House, Mitt Romney carried the state. The result was expected given that President Obama, noting Missouri’s steady shift to the right, decided not to even contest what was deemed a battleground state just four years ago. Mr. Obama narrowly lost the state to John McCain in 2008, making it the first time in nearly a century that Missouri did not back the winning presidential candidate.

In the governor’s race, the incumbent Democrat, Jay Nixon, won re-election. Mr. Nixon campaigned on his record of working with the Republican-led General Assembly to slash spending, eliminating 4,300 government jobs, and to balance the budget without raising taxes. David Spence, a St. Louis businessman, spent about $4.5 million of his own money on the race.